There is also the very popular
schiit magni 3 for about £110 on the EU schiit site (google it) its meant to provide a more 'detailed' sound over the AE-5 according to this review here:
info here:
https://forums.overclockers.co.uk/posts/32219665
But be aware even this guy who reviewed it did say he found the sound similar between AE-5 and Schiit but if you listen to music a lot you should be able to pick out the differences, I think he means the high end detail here.
Never owned a headphone amp apart from the soundblaster Z mind, ill be trying my HD650s with the AE-5 soon though.